Contact sensitive device

ABSTRACT

A contact sensitive device ( 10 ) comprising a member ( 12 ) capable of supporting bending waves, a first sensor ( 16 ) mounted on the member for measuring bending wave vibration in the member, the sensor ( 16 ) determining a first measured bending wave signal and a processor which calculates information relating to a contact on the member ( 12 ) from the measured bending wave signal, the processor applying a correction based on the dispersion relation of the material of the member ( 12 ) supporting the bending waves, characterised in that the device ( 10 ) comprises a second sensor ( 16 ) to determine a second measured bending wave signal which is measured simultaneously with the first measured bending wave signal and the processor calculates a dispersion corrected function of the two measured bending wave signals which is selected from the group consisting of a dispersion corrected correlation function, a dispersion corrected convolution function, a dispersion corrected coherence function and other phase equivalent functions to determine information relating to the contact.

TECHNICAL FIELD

[0001] The invention relates to

BACKGROUND ART

[0002] Visual displays often include some form of touch sensitivescreen. This is becoming more common with the emergence of the nextgeneration of portable multimedia devices such as palm top computers.The most established technology using waves to detect contact is SurfaceAcoustic Wave (SAW), which generates high frequency waves on the surfaceof a glass screen, and their attenuation by the contact of a finger isused to detect the touch location. This technique is “time-of-flight”,where the time for the disturbance to reach one or more sensors is usedto detect the location. Such an approach is possible when the mediumbehaves in a non-dispersive manner i.e. the velocity of the waves doesnot vary significantly over the frequency range of interest.

[0003] In contrast in WO01/48684 to the present applicant, a contactsensitive device and method of using the same are proposed. The devicecomprises a member capable of supporting bending wave vibration and asensor mounted on the member for measuring bending wave vibration in themember and for transmitting a signal to a processor whereby informationrelating to a contact made on a surface on the member is calculated fromthe change in bending wave vibration in the member created by thecontact.

[0004] By bending wave vibration it is meant an excitation, for exampleby the contact, which imparts some out of plane displacement to themember. Many materials bend, some with pure bending with a perfectsquare root dispersion relation and some with a mixture of pure andshear bending. The dispersion relation describes the dependence of thein-plane velocity of the waves on the frequency of the waves.

[0005] Two types of contact sensitive device are proposed, namely apassive sensor in which bending wave vibration in the member is onlyexcited by the contact and an active sensor in which the contactsensitive device further comprises an emitting transducer for excitingbending wave vibration in the member to probe for information relatingto the contact. In the active sensor, information relating to thecontact is calculated by comparing the response of waves generated bythe emitting transducer in the absence of a contact to the responsecaused by the mechanical constraint of the presence of a contact.

[0006] Bending waves provide advantages, such as increased robustnessand reduced sensitivity to surface scratches, etc. However, bendingwaves are dispersive i.e. the bending wave velocity, and hence the “timeof flight”, is dependent on frequency. In general, an impulse contains abroad range of component frequencies and thus if the impulse travels ashort distance, high frequency components will arrive first. This effectmust be corrected.

[0007] In WO01/48684, a correction to convert the measured bending wavesignal to a propagation signal from a non-dispersive wave source may beapplied so that techniques used in the fields of radar and sonar may beapplied to detect the location of the contact. The application of thecorrection is illustrated in FIGS. 1a to 1 d.

[0008]FIG. 1a shows an impulse in an ideal medium with a square rootdispersion relation and demonstrates that a dispersive medium does notpreserve the waveshape of an impulse. The outgoing wave (60) is evidentat time t=0 and the echo signal (62) is spread out over time, whichmakes a determination of an exact contact position problematic.

[0009] In a non-dispersive medium such as air, a periodic variation ofthe frequency response is characteristic of a reflection, and is oftenreferred to as comb filtering. Physically, the periodic variation in thefrequency response derives from the number of wavelengths that fitbetween the source and the reflector. As the frequency is increased andthe number of wavelengths fitting in this space increases, theinterference of the reflected wave with the outgoing wave oscillatesbetween constructive and destructive.

[0010] Calculating the Fourier transform of the dispersive impulseresponse of FIG. 1a produces the frequency response shown in FIG. 1b.The frequency response is non-periodic and the periodic variation withwavelength translates to a variation in frequency that gets slower withincreasing frequency. This is a consequence of the square rootdispersion in which the wavelength is proportional to the square root ofthe inverse of frequency. The effect of the panel on the frequencyresponse is therefore to stretch the response as a function of frequencyaccording to the panel dispersion. Consequently, a correction for thepanel dispersion may be applied by applying the inverse stretch in thefrequency domain, thus restoring the periodicity present in thenon-dispersive case.

[0011] By warping the frequency axis with the inverse of the paneldispersion, FIG. 1b may be transformed into the frequency response forthe non-dispersive case (FIG. 1c) in which the frequency of excitationis proportional to the inverse of the wavelength. This simplerelationship translates the periodic variation with decreasingwavelength to a periodic variation with increasing frequency as shown inFIG. 1c.

[0012] Applying the inverse Fast Fourier Transform (fft) to the trace ofFIG. 1c produces an impulse response shown in FIG. 1d which is correctedfor dispersion and where the clear reflection is restored. As is shownin FIG. 1d any particular waveshape of an impulse is preserved in timesince the waves travelling in a non-dispersive medium have a constantvelocity of travel, independent of their frequency. Accordingly, thetask of echo location is relatively straight forward. The outgoing wave(50) is evident at time t=0, together with a clear reflection (52) at 4ms. The reflection (52) has a magnitude which is approximatelyone-quarter of the magnitude of the outgoing wave (50).

[0013] The procedure described is not applicable if the impulse hasoccurred at an unknown time t₀ and the distance x from the response toan initial impulse may only be calculated if the impulse occurs at t₀=0.

[0014] It is an object of the present invention to provide analternative contact sensitive device which uses bending wave vibrationfor extracting information relating to the contact.

DISCLOSURE OF INVENTION

[0015] According to one aspect of the invention, there is provided acontact sensitive device comprising a member capable of supportingbending waves, a first sensor mounted on the member for measuringbending wave vibration in the member, the first sensor determining afirst measured bending wave signal and a processor which calculatesinformation relating to a contact on the member from the measuredbending wave signal, the processor applying a correction based on thedispersion relation of the material of the member supporting the bendingwaves, characterised in that the device comprises a second sensor todetermine a second measured bending wave signal which is measuredsimultaneously with the first measured bending wave signal and theprocessor calculates a dispersion corrected function of the two measuredbending wave signals which is selected from the group consisting of adispersion corrected correlation function, a dispersion correctedconvolution function, a dispersion corrected coherence function andother phase equivalent functions to determine information relating tothe contact.

[0016] According to a second aspect of the invention, there is provideda method of determining information relating to a contact on a contactsensitive device comprising the steps of providing a member capable ofsupporting bending waves and a first sensor mounted on the member formeasuring bending wave vibration in the member, determining, using thesensor, a first measured bending wave signal characterised by providinga second sensor mounted on the member to determine a second measuredbending wave signal, measuring the second measured bending wave signalsimultaneously with the first measured bending wave signal, calculatinga dispersion corrected function of the two measured bending wave signalswhich is selected from the group consisting of a dispersion correctedcorrelation function, a dispersion corrected convolution function, adispersion corrected coherence function and other phase equivalentfunctions and processing the measured bending wave signals to calculateinformation relating to the contact by applying the dispersion correctedfunction.

[0017] The following features may be applied to both the device and themethod with the processor being adapted to provide many of thecalculations or processing steps of the method.

[0018] The dispersion corrected function may be calculated as follows:

[0019] calculate Ŵ₁(ω) and Ŵ₂(ω)* which are the Fourier transformationand complex conjugate Fourier transformation of the two measured bendingwave signals W₁(t) and W₂(t); t represents time ω is 2πf where f isfrequency.

[0020] calculate a first intermediate function Ŵ₁(ω)Ŵ₂*(ω);

[0021] calculate a second intermediate function M(ω) which is a functionof Ŵ₁(ω)Ŵ₂*(ω);

[0022] apply a frequency stretching operation f(ω), as described abovein relation to WO01/48684, to M(ω) to give the dispersion correctedcorrelation function:${G(t)} = {\frac{1}{2\quad \pi}{\int_{- \infty}^{+ \infty}{{M\left\lbrack {f(\omega)} \right\rbrack}{\exp \left( {\quad \omega \quad t} \right)}{{\omega}.}}}}$

[0023] The intermediate function M(ω) may simply be Ŵ₁(ω)Ŵ₂*(ω) whichgives a standard dispersion corrected correlation function.Alternatively, M(ω) may be a function which modifies the amplitude butnot the phase of Ŵ₁(ω)Ŵ₂*(ω) to give a phase equivalent function to thestandard dispersion corrected correlation function. Since the phaseequivalent function and the standard dispersion corrected correlationfunction have the same phase properties, they have a maximum at the sameposition. The phase information in the measured bending wave signals maybe used to acquire information about the contact in particular thelocation thereof. The location may be calculated from the time at whichthe maximum in the functions occurs.

[0024] M(ω) may be selected from the following functions which all yieldphase equivalent functions to the standard dispersion correctedcorrelation function: $\begin{matrix}\left. a \right) & {{M(\omega)} = \frac{{{\hat{W}}_{1}(\omega)}{{\hat{W}}_{2}^{*}(\omega)}}{{{{\hat{W}}_{1}(\omega)}{{\hat{W}}_{2}^{*}(\omega)}}}}\end{matrix}$

[0025] Thus M(ω) may normalise the amplitudes of Ŵ₁(ω)Ŵ₂*(ω) to unity toyield a normalised dispersion corrected correlation function otherwiseknown as a dispersion corrected coherence function. $\begin{matrix}\left. b \right) & {{M(\omega)} = \frac{{{\hat{W}}_{1}(\omega)}{{\hat{W}}_{2}^{*}(\omega)}}{\sqrt{{{{\hat{W}}_{1}(\omega)}{{\hat{W}}_{2}^{*}(\omega)}}}}}\end{matrix}$

[0026] Thus M(ω) may act on the amplitudes of Ŵ₁(ω)Ŵ₂*(ω) to yield adispersion corrected correlation function with a modified peak shape.

[0027] c) M(ω)=Ŵ₁(ω)Ŵ₂*(ω)φ└|Ŵ₁(ω)Ŵ₂*(ω)|┘ where φ(x) is a real valuedfunction

[0028] Thus M(ω) may apply a general modification to yield a phaseequivalent function having a different amplitude to the standardcorrelation function.

[0029] d) M(ω)=Ŵ₁(ω)Ŵ₂*(ω)ψ(ω) where ψ(ω) is a real valued function

[0030] Thus M(ω) may apply a general frequency-dependent scaling toyield a phase equivalent function having a different amplitude to thestandard correlation function. Such a scaling is also known as emphasis.

[0031] Alternatively, M(ω) may be the function {circumflex over (D)}(ω)which is the Fourier transformation of the correlation function D(t):

D(t)=∫_(−∞) ^(+∞) W ₁(t+t′)W ₂(t′)dt′

[0032] {circumflex over (D)}(ω) is mathematically equivalent toŴ₁(ω)Ŵ₂*(ω) and may be arrived at without calculating Ŵ(ω) and Ŵ₂(ω)*.This is an alternative method to calculating the standard dispersioncorrected correlation function. The steps are calculate D(t); calculate{circumflex over (D)}(ω) and apply a frequency stretching operation toarrive at the dispersion corrected correlation function:${G(t)} = {\frac{1}{2\quad \pi}{\int_{- \infty}^{+ \infty}{{\hat{D}\left\lbrack {f(\omega)} \right\rbrack}{\exp \left( {\quad \omega \quad t} \right)}{{\omega}.}}}}$

[0033] One advantage of using the dispersion corrected correlationfunction is that it is applicable in situations where the precise time,t₀, at which a contact occurred is not known. This is because an offsett₀ (i.e. t₀≠0) in the response functions is represented as an additionalfactor exp(iωt₀) in the Fourier transformations, Ŵ₁(ω) and Ŵ₂(ω) whichcancels in the intermediate function Ŵ₁(ω)Ŵ₂*(ω).

[0034] A transducer may act as both the first and second sensor whereby,the dispersion corrected correlation function is an autocorrelationfunction. The autocorrelation function may be calculated applying thesame steps for the dispersion corrected correlation function usingW₁(t)=W₂(t).

[0035] The frequency stretching operation may be determined as describedin relation to FIGS. 1a to 1 d by {circumflex over (Ω)}(ω)=Ŵ[f(ω)] inwhich f(ω) is chosen so that {circumflex over (Ω)}(ω) represents animaginary response of a medium where bending waves of all frequenciestravel with identical phase velocity. For example, for bending modevibrations that follow a square-root relation of wavevector, k, toangular frequency, k=C{square root}{square root over (ω)}, the frequencystretching operation f(ω)=vC{square root}{square root over (ω)} gives animaginary response, Ω(ω), of a medium with constant arbitrary phasevelocity v. C is a constant and is defined by C=(μ/B){circumflex over( )}¼ where μ=mass per unit area, and B=bending stiffness.

[0036] The inverse Fourier transform, Ω(t) may be examined to determinethe distance to the site of the contact. Ω(t), is centred at a value t₁that is proportional to the distance between the site of the contact andeach sensor. The dispersion corrected correlation function may also beused to determine the precise location of the contact. For example, forthe function, G(t), the centre of the correlation function is located att=(x₁−x₂)/v where x₁ and x₂ are the distances from the contact point tothe respective vibration sensors. The dispersion corrected correlationfunction may therefore be used to determine a difference in path-length(i.e. x₁−x₂) between the contact site and the two sensors. Thisquantity, x₁−x₂, defines a hyperbola of possible locations of thecontact on the panel surface. If a third sensor is provided, a seconddispersion corrected correlation function, from a different combinationof sensor positions, provides a second path-length difference, eg.x₁−x₃. Alternatively, a second pair of sensors may be mounted to themember to provide a second dispersion corrected correlation function. Inmany cases (for example when the sensors are positioned in the fourcorners of a rectangular panel), the two hyperbolic curves defined bythe two path-length differences have a unique intersection point whichdetermines unambiguously the location of the contact.

[0037] Another advantage of using the dispersion corrected correlationfunction is in the treatment of waves reflected from boundaries of themember. The reflected waves create the effect of virtual sources whichare located outside the physical boundaries, specifically, at thecontact site reflected in the axes of the boundaries. The impulsegenerated by a contact may show features corresponding to ‘echoes’ whicharrive after the direct waves of the impulse. By applying the dispersioncorrection to an impulse at t=0, discrete reflections may be resolved aspeaks in the corrected impulse response. The location of these peaks maybe used as additional information for determining the location of thecontact. This may be particularly effective if the sensitivity, oracceptance, of the sensors is not independent of the direction of theincoming wave since the measured signal corresponding to the direct pathfrom the contact may be weaker than the signal due to reflected waves.

[0038] The dispersion-corrected autocorrelation function may be appliedto locate the path-length difference between the sensor and real andvirtual sources of bending waves arising from reflections at theboundaries. Such information may help the determination of the contactlocation.

[0039] Reflected waves may be suppressed by placing an absorbingmaterial in contact with the edges of the member. Alternatively, theprocessor may be adapted to remove the contribution of reflected wavesfrom the measured bending wave signal. This may be achieved on the basisthat in a dispersive medium, i.e. one with a dispersion relation of theform k=C{square root}{square root over (ω)}, low-frequency componentstravelling along the direct path may arrive after the firsthigh-frequency reflected waves. This processor may be adapted to providea moving average low-pass filtering operation on the original measuredbending wave signal where the width of the averaging window varieslocally with the time coordinate as Δt∝t².

[0040] The measured bending wave signal may be transformed to a responsein which the signals due to the direct and reflected wave both oscillatewith constant but different periods. For example, a measured bendingwave signal W(t), from a contact at time t=0 may be transformed on thetime axis by using U(τ)=W(1/τ). In the function U(τ) the signal from asharp impulse propagating on a dispersive member oscillates with aconstant period. Furthermore, the oscillations from the direct wave havea longer period than those of the reflected waves. The contribution ofreflected waves can therefore be removed by a low pass filter operationon U(τ). The response may then be transformed back to linear units oftime, as W′(t)=U(1/t). The procedure may be generalised to other formsof the dispersion relation.

[0041] For a perfect rejection of reflected waves, it is necessary toknow the dispersion relation in the member, the time t₀ at which thecontact occurred, and the distance x between the contact site and thesensor. However, generally, only the first is known. Thus the processormay be adapted to provide estimates or substitutes for t₀ and x whichmay be substituted into a calculation to remove the reflected waves. Forexample, an estimate for t₀ may be the time at which the contact wasfirst detected by any sensor on the member, i.e. the time at which themeasured signal first passes a predetermined threshold. The distance xmay be set as the distance between the relevant sensor and the furthestpoint on the member or the maximum dimension (e.g. diameter) of themember. Use of these substitutes should leave the direct-path signalintact. Estimates of t₀ and x obtained by other means may also be used.

[0042] There may be multiple (i.e. n) sensors on the member and thus thenumber of distinct correlation functions is n(n−1)/2. The processor maybe adapted to create a mapping function which maps the surface of themember for each correlation function whereby the dispersion correctedcorrelation function, G(t), is swept over the surface such that allco-ordinates of a given path-length difference, Δx, take the valueG(Δx/v). The product of the entire set of mapped correlation functionsmay then be calculated and the location of the contact may be determinedfrom the maximum co-ordinate. This method has the desired property ofincreasingly rejecting spurious contributions from reflections as thenumber of sensors is increased.

[0043] Each sensor and its associated circuitry may have identical ordifferent phase properties. Differences in the phase properties may becaused by mechanical inconsistencies in the manufacture and mounting ofthe sensors, or by finite tolerances of component values in theassociated amplification and filtering circuits. If the phase propertiesof each sensor differ, the dispersion corrected correlation function maybe convolved with some asymmetric function. This asymmetric function maylead to incorrect measurements of path-difference difference since theposition of peaks in the dispersion corrected correlation function is nolonger independent of the phase properties of the sensor.

[0044] The processor may thus be adapted to perform the following steps:

[0045] a) estimate a convolution correction coefficient φ₁₂(ω) from${\varphi_{12}^{*}(\omega)} = {{\sum\limits_{j}{{{\hat{W}}_{1,j}(\omega)}{{\hat{W}}_{2,j}^{*}(\omega)}{\exp\left\lbrack {{- }\quad {k(\omega)}\Delta \quad x_{j}} \right\rbrack}}}}$

[0046] where and {Ŵ_(1,j)(107 )} and {Ŵ*_(2,j)(107 )}are the Fouriertransformation and complex conjugate Fourier transformation of twomeasured bending wave signals {W_(1,j)(t)} and {W_(2,j)(t)} and {Δx_(j)}is the path-length difference;

[0047] b) calculate the dispersion corrected correlation function withphase correction from:${G(t)} = {\frac{1}{2\quad \pi}{\int_{- \infty}^{+ \infty}{{{\hat{W}}_{1}\left\lbrack {f(\omega)} \right\rbrack}{{\hat{W}}_{2}^{*}\left\lbrack {f(\omega)} \right\rbrack}\quad {\varphi_{12}\left\lbrack {f(\omega)} \right\rbrack}\quad {\exp \left( {\quad \omega \quad t} \right)}{{\omega}.}}}}$

[0048] The phase correction has the effect of deconvolving thedispersion corrected correlation function such that the position of themaximum is consistent with the position of the contact.

[0049] The processor may further be adapted to include in thedetermination procedure any available information about where thecontact can be expected. This may be particularly useful in situationswhere the position of a contact may not be unambiguously determined bythe dispersion corrected correlation functions, e.g. when reflectedwaves interfere with the direct-wave signature or less than threesensors are used. For example, if the member is an input device for agraphical user interface where the user is presented with a choice of‘buttons’ to press, it may be useful to assume that any contact on themember occurs within the discrete areas corresponding to the buttons.

[0050] Alternatively, a map of the probability at which a contact islikely to occur and which is based on the expected behaviour of the usermay be used. The device may comprise a software application with agraphical user interface (GUI) which interacts with the operating systemby means of an application program interface (API) in which the API isadapted to generate the probability map. The probability map may bebased on the location, size, and frequency of use of objects presentedby the graphical user interface. The probability map may also be basedon information about the relative likelihood of the various GUI elementsbeing activated.

[0051] The information in the mapped correlation functions may becombined with the probability map to give higher reliability. Theexpected probability may also be a further input to a neural-net whichaccepts information derived from the sensors.

[0052] The member may comprise a raised pattern on its surface whereby acontact drawn across the surface provides a variable force to the memberto generate bending waves in the member. The pattern may be periodic, orquasi-periodic with a statistically well-defined spatial distribution ofundulations. The processor may be adapted to determine the contactposition by using knowledge of the periodicity of the pattern and thefact that the interval between impulses represents the time in which acontact, which may be provided by a stylus, has travelled to an adjacentfeature of the pattern. The pattern may be random whereby a contacttravelling over the surface of the member generates a random bendingwave signal.

[0053] The use of random surface pattern may be used independently ofthe dispersion correction correlation function. Thus according toanother aspect of the invention, there is provided a contact sensitivedevice comprising a member capable of supporting bending waves, a sensormounted on the member for measuring bending wave vibration in the memberto determine a first measured bending wave signal and a processor whichcalculates information relating to the contact from the measured bendingwave signal from the sensor, characterised in that a surface of themembers comprises a raised pattern whereby a contact drawn across thesurface provides a variable force to the member to generate bendingwaves in the member.

[0054] The device may comprise sensing means to determine a secondmeasured bending wave signal which is measured simultaneously with thefirst measured bending wave signal and the processor may calculateinformation relating to the contact from a dispersion correctedcorrelation function of the two measured bending wave signals. Thedispersion corrected correlation function is described above and thusfeatures of the first and second embodiments may be applied to thisembodiment. The dispersion corrected correlation function isparticularly useful if the measured bending wave signals have a largebandwidth centred on a frequency which gives a phase-velocity of bendingwaves in the member which is much greater than the maximum lateralvelocity of the contact.

[0055] According to another aspect of the invention, there is provided acontact sensitive device comprising a member capable of supportingbending waves, a sensor mounted on the member for measuring bending wavevibration in the member to determine a first measured bending wavesignal and a processor which calculates information relating to acontact from the measured bending wave signal from the sensor,characterised in that the device comprises at least a second sensor todetermine a second measured bending wave signal which is measuredsimultaneously with the first measured bending wave signal and theprocessor optimises a product of a set of corrected impulse responsemeasurements from each sensor to determine information related to thecontact.

[0056] The information calculated may be the time t₀ at which contactoccurs and thus the form of the dispersion relation, k(ω), and thedistance, x, separating each sensor from the contact site must be known.The corrected impulse response measurement may be calculated by usingthe following steps:

[0057] 1) Calculate the Fourier transform Ŵ(ω) of a measured bendingwave signal W(t);

[0058] 2) Calculate an equivalent response, W′(t), from a notionalsensor positioned at the contact site by

Ŵ′(ω)=Ŵ(ω)exp[ik(107 )x].

[0059] 3) Calculate the inverse Fourier transform of Ŵ^(′)(ω) to providefunction W′(t).

[0060] The product is thus Π_(j)W′_(j)(t) in which the function W′(t)shows an initial impulse from the contact which is without dispersionand which is centred at t₀, the time of the impulse. Features in W(t)due to edge reflections will appear later in W′(t) but will not becorrected for dispersion in the same way as for the direct impulse whichis thus more easy to identify.

[0061] The product shows a strong peak due to the direct impulse, and aweak background noise. By taking the product we reinforce the signalcoincident at t₀ in all W′_(j)(t), whereas the information due toreflections is uncorrelated and suppressed. This procedure is thereforea reliable way of determining t₀.

[0062] Conversely, the information relating to the contact may be thelocation of the contact, i.e. the co-ordinate r and thus thesensor-contact distances, x_(j) and the time t₀ for which the maximumvalue of Π_(j)W′_(j)(t₀) is obtained must be known.

[0063] This optimisation process may involve iterative refinement ofestimates for r and t₀. The initial estimate may be derived from impulseresponse functions whose high temporal frequency components have beensuppressed, with the result that the estimate is easy to obtain (fewerlocal maxima), but of low spatial precision. Further iterations mayintroduce progressively higher frequency components as the estimate isrefined.

[0064] The invention therefore provides two complementary methods ofdetermining the contact position: the method of dispersion-correctedcorrelation functions, and the method of maximising Π_(j)W′_(j)(t₀).

[0065] The following characteristics may apply to all embodiments of theinvention. The device may comprise means for recording measured bendingwave signals from the or each sensor over time as the contact movesacross the member. The measured bending wave signals may be recorded astime-series data, i.e. a sequence of values measured at different times.

[0066] The time-series data may be analysed as a sequence of shortsections or ‘frames’ of data, which may have widths or durations of 10ms. The processor may apply a window function to the frames of data.Window functions are well known in the art—see for example Press W.H. etal., Numerical Recipes in C, 2 Ed., Cambridge University Press 1992, Ch.13.4. The processor may be adapted to extract information on the contactwhich has been averaged over the duration of the frame, e.g. the meanpositional co-ordinate of the moving contact. The processor may applythe dispersion corrected correlation technique to calculate the meanpath length difference for each frame of data to give a sequence ofpath-length differences over time.

[0067] Alternatively, the time-series data may be analysed usingadaptive filters such as those described in Grant PM et al “Analogue andDigital Signal Processing and Coding”, Ch 10 (1989). The adaptive filtermay correct for a convolution function which relates the time-seriesdata from the first and second measured bending wave signals. Theconvolution function changes slowly over time as the contact moves andis dependent on the position of the contact.

[0068] The adaptive filter may calculate a convolution correction whichwhen applied to the measured bending wave signals produces a signal asclose as possible to the measurement input. The first measured bendingwave signals form a first time-series data which may be sent to thereference signal of the adaptive filter and the second measured bendingwave signals form a second time-series data which may be delayed,preferably by the maximum expected width of the correlation function,before being sent to the primary input of the adaptive filter wherebythe convolution correction may be calculated. The processor may applythe dispersion correction to the convolution function to give a functionwhose maximum is determined by the difference between the path-lengthfrom contact to the sensor and the path-length from the contact to thesensing means.

[0069] The information calculated may be the location of the contact ormay be other information, e.g. pressure or size of the contact. Theinformation relating to the contact may be calculated in a centralprocessor. The sensors may be mounted at or spaced from an edge of themember. The sensors may be in the form of sensing transducers which mayconvert bending wave vibration into an analogue input signal.

[0070] The member may be in the form of a plate or panel. The member maybe transparent or alternatively non-transparent, for example having aprinted pattern. The member may have uniform thickness. Alternatively,the member may have a more complex shape, for example a curved surfaceand/or variable thickness. The processor may be adapted for complexshaped members by providing an adaptive algorithm such as a neural netto decipher the contact location from the bending wave signal receivedby the sensor.

[0071] The device may be a purely passive sensor with the bending wavevibration and hence the measured bending wave signals being generated byan initial impact or by frictional movement of the contact.Alternatively, the device may be an active sensor and thus the devicemay comprise an emitting transducer. The transducer may have dualfunctionality, namely acting as an emitting transducer and a sensor. Themember may also be an acoustic radiator and bending wave vibration inthe member may be used to generate an acoustic output.

[0072] Measurements of the bending wave signal due to a contact may becontaminated by the bending waves due to the audio signal, particularlywhen the audio signal is similar to the bending wave signal generated bythe contact. The effect may be minimised by ensuring the frequency bandof the audio signal differs from and does not overlap the frequency bandof the measurements from the sensors and sensing means. The audio andmeasured signals may thus be filtered, for example, the audio band maybe limited to frequencies below 20 kHz, and the vibration measurementsmay be limited to frequencies above 20 kHz.

[0073] The device may be a dual active and passive sensor and may beadapted to switch between active and passive sensing modes depending onwhether contact is applied to the device. The device may cycle betweenresting in passive sensing mode when no contact is detected, switchingto active mode sensing when a contact is applied and returning topassive sensing mode once the contact is removed to wait for furthercontacts. This may be advantageous to avoid the power drain when thedevice is in active mode.

[0074] The contact may be in the form of a touch from a stylus which maybe in the form of a hand-held pen. The movement of a stylus on themember may generate a continuous signal which is affected by thelocation, pressure and speed of the stylus on the member. The stylus mayhave a flexible tip, e.g. of rubber, which generates bending waves inthe member by applying a variable force thereto. The variable force maybe provided by tip which alternatively adheres to or slips across asurface of the member. As the tip moves across of the member a tensileforce may be created which at a certain threshold, causes any adhesionbetween the tip and the member to break, thus allowing the tip to slipacross the surface.

[0075] Alternatively, the contact may be in the form of a touch from afinger which may generate bending waves in the member which may bedetected by passive and/or active sensing. The bending waves may havefrequency components in the ultrasonic region (>20 kHz). Passive sensingis therefore sensitive to contacts with both fingers and styli.

[0076] When the device is acting as an active sensor, i.e. with anemitting transducer generating an excitation signal, the contact mayexert a non-linear force on the member so as to generate harmonics ofthe excitation signal. The processor may comprise signal processingdevices to isolate the excitation signal from the harmonics so that theharmonics may used to determine the contact position in a similar mannerto passive sensing. The harmonics effectively constitute a source ofbending waves from the contact site.

[0077] The or each emitting transducer or sensor may be a bendertransducer which is bonded directly to the member, for example apiezoelectric transducer. Alternatively, the or each emitting transduceror sensor may be an inertial transducer which is coupled to the memberat a single point. The inertial transducer may be either electrodynamicor piezoelectric. It may be possible to use audio transducers which arealready in place as sensing and/or emitting transducers.

[0078] A contact sensitive device according to the invention may beincluded in a mobile phone, a laptop or a personal data assistant. Forexample, the keypad conventionally fitted to a mobile phone may bereplaced by a continuous moulding which is touch sensitive according tothe present invention. In a laptop, the touchpad which functions as amouse controller may be replaced by a continuous moulding which is acontact sensitive device according to the invention. The moulding may beimplemented as a mouse controller or other alternatives, e.g. akeyboard. Alternatively, the contact sensitive device may be a displayscreen, e.g. a liquid crystal display screen comprising liquid crystalswhich may be used to excite or sense bending waves. The display screenmay present information relating to the contact.

BRIEF DESCRIPTION OF DRAWINGS

[0079] The invention is diagrammatically illustrated, by way of example,in the accompanying drawings, in which:

[0080]FIGS. 1a to 1 d are a graphic illustration of a method ofdispersion correction according to the prior art, in which FIG. 1a is agraph of a dispersive impulse response showing response in arbitraryunits against time. FIG. 1b is a graph of a dispersive frequencyresponse showing response in arbitrary units against frequency. FIG. 1cis a graph of a non-dispersive frequency response showing response inarbitrary units against frequency. FIG. 1d is a graph of anon-dispersive impulse response showing response in arbitrary unitsagainst time;

[0081]FIG. 2 is a plan view of a contact sensitive device according tothe present invention;

[0082]FIG. 3 is a perspective view of a first device incorporatingpassive touch sensing;

[0083]FIG. 4 is a perspective view of a second device incorporatingpassive touch sensing;

[0084]FIG. 5 is a block diagram of a processing algorithm for thepassive sensing of FIGS. 3 and 4;

[0085]FIG. 6 is a perspective view of first device incorporating activetouch sensing;

[0086]FIG. 7 is a perspective view of a second device incorporatingactive touch sensing;

[0087]FIG. 8 is a flow chart showing a method of calculating thepath-length difference between the contact and two measurement pointsusing the dispersion corrected correlation function;

[0088]FIG. 8a is a schematic plan view of a device to which the methodof FIG. 8 is applied;

[0089]FIG. 8b is a graph of dispersion corrected correlation functionagainst time;

[0090]FIG. 9 is a flow chart showing a first method of removingreflections from a measured bending wave signal,

[0091]FIG. 10 is a flow chart showing a second method of removingreflections from a measured bending wave signal.

[0092]FIGS. 11a and 11 b are schematic perspective and plan views of atouch sensitive device according to another aspect of the invention;

[0093]FIG. 12 is a schematic block diagram of a processing algorithmwhich may be used for the device of FIG. 11.

[0094]FIG. 12a is a graph showing the combined transfer function H(f)against frequency (f) for the filter and amplifier of FIG. 12;

[0095]FIG. 12b is a graph of a measured bending wave signal againsttime;

[0096]FIG. 13 is a flow chart of the steps for obtaining an empiricalphase correction;

[0097]FIG. 13a is a plan view of a grid defined on a member for use inthe method of FIG. 13;

[0098]FIG. 14 is a touch sensitive device according to another aspect ofthe invention;

[0099]FIG. 15 is a circuit diagram of an adaptive noise canceller whichmay be used in the various devices;

[0100]FIG. 16a is a schematic block diagram of a contact sensitivedevice which also operates as a loudspeaker;

[0101]FIG. 16b is a method of separating audio signal and measuredbending wave signal in the device of FIG. 16a;

[0102]FIG. 17 is a flow chart showing a method of calculating thecontact location using the dispersion corrected auto-correlationfunction;

[0103]FIG. 17a is a schematic plan view of a device to which the methodof FIG. 17 is applied;

[0104]FIG. 17b is a graph of dispersion corrected auto-correlationfunction against time, and

[0105]FIG. 18 is a block diagram showing how an adaptive filter may beused to calculate information relating to the contact.

[0106]FIG. 2 shows a contact sensitive device (10) comprising atransparent touch sensitive plate (12) mounted in front of a displaydevice (14). The display device (14) may be in the form of a television,a computer screen or other visual display device. A stylus (18) in theform of a pen is used for writing text (20) or other matter on the touchsensitive plate (12).

[0107] The transparent touch sensitive plate (12) is a member, e.g. anacoustic device, capable of supporting bending wave vibration. Threetransducers (16) are mounted on the plate (12). At least two of thetransducers (16) act as sensors or sensing means and are thus sensitiveto and monitor bending wave vibration in the plate. The third transducer(16) may also be a sensing transducer so that the system corresponds tothe passive contact sensitive device of FIG. 3 or FIG. 4.

[0108] Alternatively, the third transducer may be an emitting transducerfor exciting bending wave vibration in the plate so that the systemcorresponds to the active sensor of FIG. 5. In the FIG. 6 or FIG. 7embodiment, the active sensor may act as a combined loudspeaker andcontact sensitive device.

[0109]FIGS. 3 and 4 are more detailed illustration of two contactsensitive devices (32,33). The contact sensitive devices (32,33)comprises a member in the form of a panel (24) capable of supportingbending wave vibration and three sensors in the form of sensingtransducers (26) for sensing bending wave vibration at their respectivemounting points. The vibration pattern (28) is created when pressure isapplied at a contact point (30). The devices may be considered to bepassive contact sensitive devices since the devices do not comprise anemitting transducer. Thus the bending wave panel vibration in the panelis generated solely by the contact.

[0110] In a passive sensor an impulse in the body of the panel (24)starts a bending wave travelling towards the edge of the panel (24). Thebending wave is detected by the three sensing transducers (26) mountedequidistantly around the edges as in FIG. 3 or by the three sensingtransducer mounted on a surface of the panel (24) but spaced from theedges of the panel (24) as in FIG. 4. The measured bending wave signalsare processed to determine the spatial origin and force profile of theapplied impulse.

[0111]FIG. 5 shows a possible implementation for the processing of thebending wave information sensed at each sensing transducer (26) of FIG.3 or FIG. 4. In FIG. 5, the bending waves in the panel are sensed bythree sensing transducers (26). The sensing transducers (26) measureanalogue bending wave signals W₁(t), W₂(t) and W₃(t) which aretransmitted to a multiplexing analogue to digital converter (ADC) (54).The resultant digital input signal is transmitted to the centralprocessor (34) from which information (58) relating to the location andprofile of the contact impulse is determined.

[0112]FIGS. 6 and 7 are more detailed illustrations of alternativecombined touch sensitive and audio devices (35,37). The devices eachcomprise a panel (24) capable of supporting bending wave vibration andan emitting transducer (31) for exciting bending wave vibration in thepanel (24). The device (35) in FIG. 6 further comprises two sensingtransducers (26) for sensing bending wave vibration at their respectivemounting points whereas the device (37) in FIG. 7 comprises only onesensing transducer (26). The vibration pattern (28) is interrupted whenpressure is applied at a contact point (30). The devices may beconsidered to be active contact sensitive devices since the devicescomprise an emitting transducer (31).

[0113] In FIG. 6, the sensing and emitting transducers (26,31) arespaced equidistantly around the edges of the panel (24) whereas in FIG.7, the sensing and emitting transducers (26,31) are distanced from theedges of the panel (24) and are mounted to a surface thereof.

[0114]FIG. 8a shows an embodiment having two sensors (102) mounted on amember (100) to which a contact is applied at a contact location (104).FIG. 8 shows a method of calculating the dispersion correctedcorrelation function to reveal the difference in path length between thecontact location (104) and the sensors (102). The method comprises thefollowing steps:

[0115] (a) Measure two bending wave signals W₁(t) and W₂(t);

[0116] (b) Remove reflections from the measured signals and calculateW′₁(t) and W′₂(t),e.g. by using the method set out in FIG. 9;

[0117] (c) Calculate the Fourier transform of W′₁(t) and W′₂(t) toarrive at Ŵ₁(ω) and Ŵ₂(ω) and hence the intermediate functionŴ₁(ω)Ŵ₂*(ω); where Ŵ₂*(ω) is the complex conjugate Fourier transform.

[0118] (d) and (e) at the same time as performing steps (a) to (c), thefrequency stretching operation f(ω)=vC{square root}{square root over(ω)} is calculated using the predetermined panel dispersion relationk=C{square root}{square root over (ω)}.

[0119] (f) Ŵ₁(ω) and Ŵ₂(ω) and f(ω)=vC{square root}{square root over(ω)} are combined to arrive at the dispersion corrected correlationfunction:${{G(t)} = {\frac{1}{2\quad \pi}{\int_{- \infty}^{+ \infty}{{{\hat{W}}_{1}\left\lbrack {f(\omega)} \right\rbrack}{{\hat{W}}_{2}^{*}\left\lbrack {f(\omega)} \right\rbrack}\quad {\exp \left( {\quad \omega \quad t} \right)}{\omega}}}}};$

[0120] and

[0121] (g) the dispersion corrected correlation function is plottedagainst time with a peak occurring at time t₁₂ as shown in FIG. 8b;

[0122] (h) Δx₁₂ is calculated from t₁₂; Δx₁₂ is the path-lengthdifference between the path lengths x₁ and x₂ from the first and secondsensors to the contact.

[0123] (i) Δx₁₂ is used to calculate the location of the contact.

[0124] Alternatively at step (e), the dispersion corrected correlationfunction with phase correction φ₁₂ set out below may be used. Thecalculation of φ₁₂ is explained in FIG. 13.${G(t)} = {\frac{1}{2\quad \pi}{\int_{- \infty}^{+ \infty}{{{\hat{W}}_{1}\left\lbrack {f(\omega)} \right\rbrack}{{\hat{W}}_{2}^{*}\left\lbrack {f(\omega)} \right\rbrack}\quad {\varphi_{12}\left\lbrack {f(\omega)} \right\rbrack}\quad {\exp \left( {\quad \omega \quad t} \right)}{\omega}}}}$

[0125]FIG. 9 shows a method of removing reflections from an impulsemeasurement where the impulse occurs at t=0. The method comprises thefollowing steps:

[0126] i) Measure a bending wave signal W₁(t);

[0127] ii) Transform the signal on the time axis by using U(τ)=W(1/τ);

[0128] iii) A low pass filter is applied, for example, U′(τ) as shown toremove all reflected signals. The constant C from the predeterminedpanel dispersion relation k=C{square root}{square root over (ω)} is usedto define the width of the convolution function Δτ;

[0129] iv) The response may then be transformed back to linear units oftime, as W′(t)=U(1/t).

[0130]FIG. 10 shows an alternative method for removing reflections froman impulse measurement where the impulse occurs at t=0. The methodcomprises the following steps:

[0131] i) Measure a bending wave signal W₁(t);

[0132] ii) Estimate the distance x between each sensor and the locationof the contact;

[0133] iii) Use the estimate x and a predetermined dispersion relationk=C{square root}{square root over (ω )}to define an averaging windowN(t, t′)

[0134] iv) Apply the averaging window N(t,t′) to the bending wave signalW₁(t) to remove the effect of reflections.

[0135] N(t,t′) is an example of an averaging window with a Gaussianshape. A rectangular window can be implemented to give computationalefficiency. Multiple applications of a rectangular window may bedesirable; a large number of repeated applications of a rectangularwindow will produce a similar result to N(t, t′).

[0136]FIGS. 11a and 11 b show a contact sensitive device (80) comprisinga rectangular member (82) capable of supporting bending waves and foursensors (84) for measuring bending wave vibration in the member. Thesensors (84) are in the form of piezoelectric vibration sensors and aremounted on the underside of the member (82), one at each corner. A foammounting (86) is attached to the underside of the member and extendssubstantially around the periphery of the member. The foam mounting (86)has adhesive surfaces whereby the member may be securely attached to anysurface. The foam mounting may reduce the reflections from the edge ofthe member.

[0137] Two sets of path length differences, Δx₁₂=x₁−x₂ and Δx₃₄=x₃−x₄are calculated as described in FIG. 8; x_(i) is the distance from eachsensor to the contact. As shown in FIG. 11b the hyperbolae (85) definedby the two path-length differences are plotted and the location of thecontact is the intersection (87) of two hyperbolae.

[0138]FIG. 12 is a schematic diagram illustrating the implementation ofa processing algorithm in the device of FIG. 11. The sensors (84)measure analogue bending wave signals W₁(t), W₂(t) and W₃(t) which arepassed through an amplifier and anti-aliasing (low-pass) filter (88).The amplitude of the combined transfer function H(t) of the amplifierand anti-aliasing filter is shown in FIG. 12a. The filtered signals areconverted into digital signals by a digitiser (90) and stored in afirst-in-first-out buffer having finite length. The buffer comprises twostores, a pre-trigger and a post-trigger store (92,93) for signalsmeasured before and after the detection process is triggeredrespectively.

[0139] The central processor (94) determines information relating to thelocation and profile of a contact of the member by the following steps:

[0140] a) The central processor performs a threshold test which isillustrated in FIG. 12b. The measured bending wave signal (96) iscompared to a predetermined threshold value (98). When the measuredsignal passes the threshold value, the detection process is triggered.

[0141] b) An array of time-series digital input signals is transferredfrom the buffer to the processor. The signals include measurements takenbefore and after the detection process is triggered so that a digitalmeasure of the entire waveform of the impulse from the contact isreconstructed.

[0142] c) The processor shifts the waveform according to the estimate oft₀ so that t₀ is set to zero.

[0143] d) The processor removes the effect of reflections from thedigitised signal as described above with the estimate of t₀ as zero andthe estimate of x taken as the diagonal length of the member.

[0144] e) The processor applies further processing, in particularcalculating the dispersion corrected correlation function for eachdiagonally opposed pair of sensors and calculating information relatingto the contact.

[0145] The further processing applied by the processor may also compriseapplying a pre-determined phase correction to the dispersion correctedcorrelation function. This may be calculated as shown in FIG. 13.

[0146] a) Define a grid—for example, for the embodiment shown in FIGS.11a and 11 b which has a member (82) which four sensors (84), this maybe done by defining points (71) by the vector {r_(j)}. In this examplethe grid has 8 rows and 7 columns so the grid is defined by a set of 56vectors [r₁, r₂ . . . r₅₆].

[0147] b) A user taps on the first point (71) in the grid defined by r₁and the first and second bending wave signals {W_(1,j)(t)} and{W_(2,j)(t)} are measured by the first and second sensor of each pair ofsensors;

[0148] c) Step (b) is repeated until the user has tapped on each pointin the grid;

[0149] d) Calculate the Fourier transforms of the bending wave signals;

[0150] e) Calculate the empirical phase correction:${\varphi_{12}^{*}(\omega)} = {{\sum\limits_{j}{{{\hat{W}}_{1,j}(\omega)}{{\hat{W}}_{2,j}^{*}(\omega)}{\exp\left\lbrack {{- }\quad {k(\omega)}\Delta \quad x_{j}} \right\rbrack}}}}$

[0151] where Δx_(j) is the difference between the path lengths, x_(1,j)& x_(2,j) from the first and second sensors to the contact. The pathlengths are known from the grid coordinates.

[0152]FIG. 14 shows a contact sensitive device (70) comprising a member(72) capable of supporting bending waves and three sensors (64) mountedon the member for measuring bending wave vibration in the member. Asurface of the member (72) comprises a raised pattern (66) which is aperiodic pattern of raised crossed lines. A stylus (78) is drawn acrossthe surface along a path (74) and as it crosses a line of the pattern itgenerates bending waves (76) in the member.

[0153]FIG. 15 shows an adaptive noise canceller for example as describedin “Widrow et al Adaptive Noise Cancelling: Principles and Applications,Proceedings of the IEEE, Vol 63 No 12 pp 1692 (1975)”. The adaptivenoise canceller comprises an adaptive filter (40) which takes theinitial audio signal as the reference input. The canceller may be usedto remove the contribution of the audio signal from the output of avibration sensor before any further processing occurs. The adaptivefilter shown is one example of an adaptive filter that can be applied tothis task.

[0154]FIG. 16a shows a contact sensitive device which also operates as aloudspeaker. FIG. 16b shows a method for partitioning the audio signaland measured signal into two distinct frequency bands so that thecontribution of the audio signal to the processed measured signal issuppressed. The device comprises a member (106) in which bending wavesare generated by an emitting transducer or actuator (108) and thecontact. The emitting transducer applies an audio signal to the member(106) to generate an acoustic output. Before being applied to themember, the audio signal is filtered by a low pass filter (112) which,as shown in FIG. 16b, removes the audio signal above a thresholdfrequency f₀.

[0155] As shown in FIG. 16b, the contact generates a signal which has apower output which is substantially constant over a large frequencyband. The signal from the contact and the audio signal sum to give acombined signal which is passed through a high pass filter (114) toremove the signal above the threshold frequency f₀. The filtered signalis then passed to a digitiser (116) and onto a processor (118).

[0156]FIG. 17a shows an embodiment having a single sensor (120) mountedon a member (100) to which a contact is applied at a contact location(104). Bending waves are reflected from the edge of the member andcreate an image of a virtual source which is at location (122). FIG. 17shows a method of calculating the dispersion corrected auto-correlationfunction to reveal the contact location (104). The method comprises thefollowing steps:

[0157] (a) Measure one bending wave signal W₁(t);

[0158] (b) Calculate the Fourier transform of W′₁(t) to arrive at

[0159] (c) at the same time as performing steps (a) and (b), thefrequency stretching operation f(ω)=vC{square root}{square root over(ω)} is calculated using the predetermined panel dispersion relationk=C{square root}{square root over (ω)}.

[0160] (d) Ŵ₁(ω) and f(ω)=vC{square root}{square root over (ω)} arecombined to arrive at the dispersion corrected auto-correlationfunction:${{G(t)} = {\frac{1}{2\quad \pi}{\int_{- \infty}^{+ \infty}{{{\hat{W}}_{1}\left\lbrack {f(\omega)} \right\rbrack}{{\hat{W}}_{2}^{*}\left\lbrack {f(\omega)} \right\rbrack}\quad {\exp \left( {\quad \omega \quad t} \right)}{\omega}}}}};$

[0161] and

[0162] (g) the dispersion corrected correlation function is plottedagainst time with peaks occurring at time t₁₁ and −t₁₁ as shown in FIG.17b;

[0163] (h) Δx₁₁ is calculated from t₁₁; Δx₁₂ is the path-lengthdifference between the path lengths x₁ and x₁′ from the first and secondsensors to the contact.

[0164] (i) Δx₁₂ is used to calculate the location of the contact.

[0165]FIG. 18 shows the adaptive filter of FIG. 15 may also be used tocalculate the location of the contact from a device comprising twosensors. In general, adaptive filters contain a finite-impulse-response(FIR) filter. A FIR filter is equivalent to a convolution operation,with some convolution function Φ(t).

[0166] The signals W₁(t) measured by the first sensor are sent to theadaptive filter (40) and the signals W₂(t) measured by the second sensorare sent to a delay unit (41). The delay unit delays the signals fromthe second sensor, preferably by the maximum expected width of theconvolution function. The delayed signals are then sent to the primaryinput of the adaptive filter. The adaptive filter continually updatesthe convolution function so that an estimate, {tilde over (W)}₂(t), ofthe primary signal input, W₂(t), may be obtained from the referenceinput, W₁(t). The convolution operation is defined as follows:

{tilde over (W)} ₂(t)=∫_(−∞) ^(+∞) W ₁(t′)Φ(t−t′)dt′.

[0167] The location of the contact is calculated in the processor by thefollowing steps:

[0168] a) Extract the FIR convolution function from the internal memoryof the adaptive filter.

[0169] b) Calculate the Fourier transform of the FIR convolutionfunction.

[0170] c) Apply the frequency stretching operation f(ω)=vC{squareroot}{square root over (ω)};

[0171] d) Calculate the inverse Fourier transform to arrive at F(t).

[0172] F(t) is a phase equivalent of the dispersion correctioncorrelation function G(t) and thus the Fourier transforms of G(t) andF(t) have equal phase but not necessarily the same amplitude.Accordingly, the location of any peaks in the time domain for F(t) andG(t) are the same and thus the location of is the contact may becalculated from the peak of F(t) as described above for G(t).

1. A contact sensitive device comprising a member capable of supportingbending waves, a first sensor mounted on the member for measuringbending wave vibration in the member, the sensor determining a firstmeasured bending wave signal and a processor which calculatesinformation relating to a contact on the member from the measuredbending wave signal, the processor applying a correction based on thedispersion relation of the material of the member supporting the bendingwaves, characterised in that the device comprises a second sensor todetermine a second measured bending wave signal which is measuredsimultaneously with the first measured bending wave signal and theprocessor calculates a dispersion corrected function of the two measuredbending wave signals which is selected from the group consisting of adispersion corrected correlation function, a dispersion correctedconvolution function, a dispersion corrected coherence function andother phase equivalent functions to determine information relating tothe contact.
 2. A contact sensitive device according to claim 1, whereinthe first sensor acts as the second sensor whereby the calculateddispersion corrected function is an autocorrelation function.
 3. Acontact sensitive device according to claim 1 or claim 2, comprising asecond pair of sensors to determine two additional measured bending wavesignals from which a second dispersion corrected function is calculated.4. A contact sensitive device according to claim 3, wherein theprocessor determines from each dispersion corrected function a firstdifference in path-length between the contact site and each of the firstand second sensors and a second difference in path-length between thecontact site and each of the second pair of sensors and determines thelocation of the contact from the first and second differences inpath-length.
 5. A contact sensitive device according to any precedingclaim, comprising absorbers mounted around at least part of theperiphery of the member to absorb reflected waves.
 6. A contactsensitive device according to any preceding claim, wherein the processoris configured to remove the contribution of reflected waves from themeasured bending wave signal.
 7. A contact sensitive device according toclaim 6, wherein the processor comprises a low-pass filtering operatorwhich operates on the measured bending wave signal and which comprisesan averaging window having a width which varies locally with time.
 8. Acontact sensitive device according to claim 6 or claim 7, wherein theprocessor is configured to provide an estimate for the distance betweenthe contact site and each sensor, the estimate being substituted into acalculation to remove the reflected waves.
 9. A contact sensitive deviceaccording to any preceding claim, comprising multiple sensors on themember whereby multiple dispersion corrected functions are determined.10. A contact sensitive device according to claim 9, wherein theprocessor is configured to create a mapping function which maps thesurface of the member for each dispersion corrected function.
 11. Acontact sensitive device according to any one of the preceding claims,wherein the member is an acoustic radiator and an emitting transducer ismounted to the member to excite bending wave vibration in the member togenerate an acoustic output.
 12. A contact sensitive device according toclaim 11, comprising means ensuring that the acoustic output andmeasured bending wave signals are in discrete frequency bands.
 13. Acontact sensitive device according to claim 12, comprising an adaptivenoise canceller for removing the contribution of the acoustic outputfrom the measured bending wave signal.
 14. A contact sensitive deviceaccording to any one of the preceding claims, wherein the member istransparent.
 15. A contact sensitive device according to any one of thepreceding claims, wherein the processor estimates a convolutioncorrection coefficient which is applied to the dispersion correctedfunction thereby compensating for phase differences between the sensors.16. A contact sensitive device according to any one of the precedingclaims, wherein the member comprises a raised pattern on its surfacewhereby a contact drawn across the surface provides a variable force tothe member to generate bending waves in the member.
 17. A contactsensitive device according to claim 16, wherein the pattern is randomwhereby random bending wave vibration is generated in the member.
 18. Acontact sensitive device according to any one of the preceding claims,comprising means for recording sets of first and second measured bendingwave signals from the or each sensor over time as the contact movesacross the member.
 19. A contact sensitive device according to claim 18,wherein the processor is adapted to analyse the measured bending wavesignals as a sequence of frames of data.
 20. A contact sensitive deviceaccording to claim 18, wherein the processor is adapted to extractinformation on the contact which has been averaged over the duration ofthe frame.
 21. A contact sensitive device according to any one of thepreceding claims, wherein the processor comprises an adaptive filterwhich calculates a convolution function between the set of firstmeasured bending wave signals and the set of second measured bendingwave signals.
 22. A contact sensitive device according to claim 21,wherein the processor is adapted to use a dispersion correctedconvolution function to calculate information about the contact.
 23. Amethod of determining information relating to a contact on a contactsensitive device comprising the steps of providing a member capable ofsupporting bending waves and a first sensor mounted on the member formeasuring bending wave vibration in the member, determining, using thesensor, a first measured bending wave signal characterised by providinga second sensor mounted on the member to determine a second measuredbending wave signal, measuring the second measured bending wave signalsimultaneously with the first measured bending wave signal, calculatinga dispersion corrected function of the two measured bending wave signalswhich is selected from the group consisting of a dispersion correctedcorrelation function, a dispersion corrected convolution function, adispersion corrected coherence function and other phase equivalentfunctions and processing the measured bending wave signals to calculateinformation relating to the contact by applying the dispersion correctedfunction.
 24. A method according to claim 23, wherein the dispersioncorrected correlation function is calculated by the following steps:calculate the Fourier transformation and complex conjugate Fouriertransformation of the two measured bending wave signals; calculate anintermediate function by multiplication of the Fourier transformationand complex conjugate Fourier transformation of the two bending wavesignals; combine a frequency stretching operation and subsequent inverseFourier transformation with the intermediate function to give thedispersion corrected correlation function.
 25. A method according toclaim 23, wherein a phase equivalent function is calculated by thefollowing steps: calculate the Fourier transformation and complexconjugate Fourier transformation of the two measured bending wavesignals; calculate a first intermediate function by multiplication ofthe Fourier transformation and complex conjugate Fourier transformationof the two bending wave signals; calculate a second intermediatefunction which is a function of the intermediate function; combine afrequency stretching operation and subsequent inverse Fourier transformwith the second intermediate function to give the phase equivalentfunction.
 26. A method according to claim 25, wherein the secondintermediate function is calculated by normalising the firstintermediate function to give the dispersion corrected coherencefunction.
 27. A method according to any one of claims 24 to 26,comprising removing the effect of reflections before calculating the oreach intermediate function.
 28. A method according to any one of claims23 to 27, comprising using the dispersion corrected function todetermine a difference in path-length between the contact site and eachsensor and hence to determine the location of the contact.
 29. A methodaccording to any one of claims 23 to 28, comprising providing atransducer which acts as both the first and second sensor andcalculating a dispersion corrected autocorrelation function.
 30. Amethod according to any one of claims 23 to 29, comprising providing asecond pair of sensors to determine two additional measured bending wavesignals from which a second dispersion corrected function is calculated.31. A method according to claim 30, comprising determining from eachdispersion corrected function a first difference in path-length betweenthe contact site and each of the first and second sensors and a seconddifference in path-length between the contact site and each of thesecond pair of sensors and determining the location of the contact fromthe first and second differences in path-length.
 32. A method accordingto any one of claims 23 to 31, comprising removing the contribution ofreflected waves from each measured bending wave signal by operating alow-pass filtering operation on the measured bending wave signals.
 33. Amethod according to any one of claims 23 to 31, comprising providingmultiple sensors on the member whereby multiple dispersion correctedfunctions are determined.
 34. A method according to claim 33, comprisingcreating a mapping function which maps the surface of the member foreach dispersion corrected function.
 35. A method according to any one ofclaims 23 to 34, comprising calculating a convolution correctioncoefficient and applying the convolution correction coefficient whencalculating the dispersion corrected function to compensate for anyphase differences between the sensors.
 36. A method according to any oneof claims 23 to 35, comprising recording a set of first measured bendingwave signals and a set of second measured bending wave signals from theor each sensor over time as the contact moves across the member.
 37. Amethod according to claim 36, comprising analysing the sets as asequence of frames of data.
 38. A method according to claim 37,comprising extracting information relating to the contact from eachframe which has been averaged over the duration of the frame.
 39. Amethod according to claim 37 or claim 38, comprising correcting for aconvolution function between the sets.
 40. A contact sensitive devicecomprising a member capable of supporting bending waves, a sensormounted on the member for measuring bending wave vibration in the memberto determine a first measured bending wave signal and a processor whichcalculates information relating to a contact from the measured bendingwave signal from the sensor, characterised in that the device comprisesat least a second sensor to determine a second measured bending wavesignal which is measured simultaneously with the first measured bendingwave signal and the processor optimises a product of a set of correctedimpulse response measurements from each sensor to determine informationrelated to the contact.
 41. A contact sensitive device according toclaim 40, wherein the corrected impulse response measurement iscalculated by using the following steps: Calculate the Fourier transformof the measured bending wave signal; Calculate an equivalent responsefrom a notional sensor positioned at the contact site Calculate theinverse Fourier transform of the equivalent response to provide afunction to be optimised.
 42. A contact sensitive device according toclaim 41, wherein the optimisation process involve iterative refinementof estimates of the location of the contact and the time for which themaximum value of the product is obtained.
 43. A contact sensitive deviceaccording to claim 42, wherein an initial estimate is derived fromimpulse response functions whose high frequency components have beensuppressed.
 44. A contact sensitive device comprising a member capableof supporting bending waves, a sensor mounted on the member formeasuring bending wave vibration in the member to determine a firstmeasured bending wave signal and a processor which calculatesinformation relating to the contact from the measured bending wavesignal from the sensor, characterised in that a surface of the memberscomprises a raised pattern whereby a contact drawn across the surfaceprovides a variable force to the member to generate bending waves in themember.
 45. A contact sensitive device according to claim 44, whereinthe pattern is periodic.
 46. A contact sensitive device according toclaim 45, wherein the pattern is quasi-periodic with a statisticallywell-defined spatial distribution of undulations.
 47. A contactsensitive device according to any one of claims 40 to 46, wherein theprocessor is configured to determine the contact position by usingknowledge of the periodicity of the pattern and the fact that theinterval between impulses represents the time in which a contact hastravelled to an adjacent feature of the pattern.
 48. A contact sensitivedevice according to any one of claims 1 to 22 and 40 to 48, wherein thedevice is a purely passive sensor with the bending wave vibration andhence the measured bending wave signals are generated by an initialimpact or by frictional movement of the contact.
 49. A contact sensitivedevice according to any one of claims 1 to 22 or 40 to 48, wherein thedevice is an active sensor comprising an emitting transducer.
 50. Acontact sensitive device according to claim 48 or claim 49, wherein thedevice is a dual active and passive sensor and is configured to switchbetween active and passive sensing modes depending on whether contact isapplied to the device.
 51. A contact sensitive device according to claim50, wherein the device cycles between resting in passive sensing modewhen no contact is detected, switching to active mode sensing when acontact is applied and returning to passive sensing mode once thecontact is removed to wait for further contacts.